Complete newbie to linux question

Hello everybody,
I've recently decided to completely reformat my old desktop and install Centos on it to create The Perfect Server. One thing that I did differently than specified in that guide was to leave DHCP enabled in the setup process. So, I followed that guide, got ISPConfig 3 up and running, and now here's my question: How do I link a domain that I purchased (from fatcow.com) to my ISPConfig3 control panel? What I've done so far is use fatcow's DNS tool to map my home external ip address (68.xxx.xxx.xxx) to the purchased domain. So now when I go to mydomain.com I get the Apache index page. That's great! But the only way that I can access ISPConfig3 is by typing my internal ip address (192.xxx.xxx.xxx) followed by :8080. What needs to be configured to make ISPConfig3 accessible through my purchased domain? I hope this isn't too confusing, I couldn't find anything on the forums that dealt with this specifically. Thanks for any help you can provide!